var items = document.getElementsByClassName('item');
//循环

var index = 0;

var clearActive = function () {
    for(var i =0;i<items.length;i++){
        items[i].className = 'item';  
    } 
}
var goIndex = function () {
    clearActive();
    items[index].className = 'item active';
}

var goNext = function(){
    if (index < 4 ) {
        index ++;
    }
    else
    index = 0;
    goIndex();
}
var goPre =function(){
    if (index == 0) {
        index = 4;
    }
    else{
        index --;
    }
    goIndex();
}

var goPreBtn = document.getElementById('left');
var goNextBtn = document.getElementById('right');

goPreBtn.addEventListener('click',function () {
    goPre();
})
goNextBtn.addEventListener('click',function () {
    goNext();
})